Usefulness is a test for conservation
The quest for immortality has been transferred from humans themselves (cryogenics notwithstanding) to the apparently more permanent world of buildings. No matter how much we mortals may change and decay, architecture can be preserved forever in a sea of art history and chemicals, eternal tribute to yesteryear's desires and values. Preservation is now promoted under all variety of banners: the power station can be both art gallery and Disneyworld; the old church can be bijou apartments, night-club or designer's office. It's just so green to re-use these old turkeys; it's so unsettling to change the skyline too abruptly. And, let's face it, who would want to trade a cinema for a parking lot?
The heritage brigade, broadly speaking, want to save things at any cost (except to themselves). They represent, politically, that stream of authoritarian government which goes round telling people what is good for them. Currently they feel very comfortable with a government including the beef-on-the- bone charlatan, 'Dr' Jack Cunningham, whose continued presence in government is a tribute to genuinely random distribution of the political survival gene.
There is a simple test as to whether a building is worth spending large amounts of money on: what will it be used for when it is 'saved' and who will pay for its continuing upkeep? The further down the architectural food chain you go, the more difficult it is to justify taxpayer expenditure. Proper recording and documentation become increasingly attractive.
Pimlico School should be saved, because it is a functioning school, the parents and teachers want it kept in use, and it represents a form of architecture we are unlikely to see again. Set against that Westminster Council's get-rich-quick pfi scheme (luxury flats and another school - eventually), and there is little contest. The school should be kept in use, rather than becoming part of Westminster's sorry history of property development, chiefly remembered for its novel valuation of cemeteries.